This invention generally relates to alloys used as coatings for gas turbine engine components and, more particularly, to coatings of MCrAlY alloys that are resistant to hot corrosion oxidation and sulfidation at high temperatures.
Components of gas turbines such as blades and vanes are often constructed from high strength alloys. However, many of these materials, such as the nickel-cobalt based alloys, are susceptible to high-temperature oxidation and corrosion.
Accordingly, it is a normal practice to coat the turbine components with oxidation and corrosion resistant materials such as the MCrAlY alloys, as exemplified by U.S. Pat. Nos. 3,649,225; 3,676,085; 3,754,903; 3,918,139; 4,005,989; 4,101,715 and 4,214,042. Typically, the MCrAlY coatings comprise small proportions of yttrium (on the order of 1-2%), relatively larger proportions of chromium and aluminum (on the order of 15-40% and 10-25% respectively), and the remaining balance selected from the group of cobalt, nickel or iron and represented by the letter M. The MCrAlY coatings are normally applied as overlay coatings in which the MCrAlY alloy is deposited on the substrate by various techniques such as vacuum vapor deposition, sputtering, and plasma spray deposition as disclosed, for example, in U.S. Pat. Nos. 3,873,347; 4,101,713; 4,101,715; 4,145,481; 4,152,488 and 4,198,442.
It has also been suggested that additional coating improvements are possible through the use of multiple coating layer and composite coatings. For example, U.S. Pat. No. 3,649,225 describes a coating comprising a chromium rich interlayer interposed between an alloy substrate and an aluminized MCrAlY overlayer. Another "stratified" coating is disclosed in U.S. Pat. No. 4,005,989 wherein an aluminide interlayer is disposed between a nickel/cobalt substrate and an MCrAlY overlayer. Composite coatings employing platinum group metals are disclosed in U.S. Pat. Nos. 3,677,789; 3,819,338; 3,829,969 and 3,918,139. For example, U.S. Pat. Nos. 3,677,789; and 3,819,338 disclose a coating process for nickel and/or cobalt alloy substrates wherein a thin platinum layer is deposited on the substrate followed by diffusion of aluminide into the platinum layer.